As a system for projecting television or video images, or as a system for use in presentations using a personal computer, there have been known image display devices such as liquid crystal projectors and digital light processing (DLP) projectors. The image display devices of these types employ as a light source a high-pressure mercury lamp with a short arc structure, which is nearly identical to a point light source.
In general, a high-pressure mercury lamp includes a transparent envelope in which a pair of tungsten electrodes are arranged so as to substantially face each other, and mercury, halogen (e.g., bromine), and a rare gas are sealed. Halogen is sealed because it causes tungsten, as the electrode material, having evaporated from the electrodes heated to high temperatures to be re-deposited on the electrodes during lighting (this phenomenon is referred to as “halogen cycle”), thereby preventing the blackening of an inner wall of the transparent envelope due to the adhesion of the tungsten.
It is generally well known that the halogen cycle is accelerated when a trace amount of oxygen (O) is present in addition to halogen atoms X. This is because WOnXm has a higher saturated vapor pressure than compounds represented by WnXm. However, it also is known that when an impure gas in the molecular state, e.g., oxygen (O2), is present in the transparent envelope, the starting voltage of the lamp becomes high, which may cause a failure in starting the lamp.
Furthermore, the following problem also is known. When a considerable amount of oxygen is present with halogen in the transparent envelope, the tungsten electrodes are oxidized to accelerate the evaporation of the tungsten during lighting. The tips of the electrodes thus are eroded and/or deformed more considerably. As a result, the distance between the electrodes varies from the original, thereby deteriorating lamp characteristics. More specifically, because the distance between the electrodes becomes longer, increasing the luminance by adopting the short arc structure as originally intended becomes impossible.
On this account, in high-pressure discharge lamps including a high-pressure mercury lamp, in order to prevent an impure gas such as oxygen (O2) from being present in a transparent envelope, processes for actively removing the impure gas have been performed conventionally. Specifically, as a material of a transparent envelope, high-purity quartz glass with an OH group content of not more than 5 ppm is used, for example. Furthermore, the quartz glass that has been formed and processed into the transparent envelope is heated at a high temperature in a vacuum to remove water (H2O) impregnated into the quartz glass by a gas burner used when forming and processing the quartz glass. Also, electrodes are subjected to a hydrogen-reducing treatment to remove gases therefrom and/or heated at a high temperature in a vacuum, before being sealed in the transparent envelope. Furthermore, the process for sealing the electrodes in the transparent envelope is performed, for example, in an argon gas atmosphere, in order to prevent the electrodes from being oxidized by heating during this sealing process.
As described above, conventional high-pressure mercury lamps have been produced in such a manner that oxygen present in a transparent envelope is minimized. On the other hand, in order to prevent the blackening of an inner wall of the transparent envelope from occurring, reducing potassium (K), which is a factor inhibiting the halogen cycle, contained in components of a lamp, e.g., electrodes, has been proposed, as disclosed in JP 11(1999)-149899A, for example.
By the way, conventional high-pressure mercury lamps have a rated life of about 2000 hours. However, in recent years, long-life high-pressure mercury lamps having a rated life of 5000 hours are demanded for use as a light source in rear TVs, for example.
However, even the conventional high-pressure mercury lamps using high-purity electrodes with reduced content of potassium cannot achieve such a long life because the blackening of an inner wall of the transparent envelope, especially at portions near the electrodes, occurs after 3000 hours of lighting. The blackening causes the transparent envelope to be heated abnormally, which may result in the breakage of the transparent envelope.
Furthermore, some image display devices (projectors) have a light control function according to the following two modes: a normal mode in which a high-pressure mercury lamp is operated at a rated power (e.g., 220 W) and an energy-saving mode in which a power lower than the rated power is input to the high-pressure mercury lamp to make the luminance lower than in the normal mode. However, according to the conventional high-pressure mercury lamps, the following problem arises. That is, when such a light control function is used, i.e., when the high-pressure mercury lamps are operated at a power lower than the rated power, the blackening of an inner wall of the transparent envelope, especially at portions near the electrodes, becomes considerable as compared with the case where the lamps are operated at the rating power, so that the life of the lamps is shortened when operated in the energy-saving mode (i.e., at the lower power).